1. Field of the Invention
The present invention relates to an improved catalyst for use in the oxidation of n-butane to maleic anhydride. In particular, the present invention is directed to the method of making metal promoted oxidation catalysts comprising the mixed oxides of vanadium and phosphorus, wherein the catalysts are promoted with at least one of niobium, cobalt, iron, zinc, molybdenum or titanium. This preparation method yields improved catalytic compositions for the oxidation of n-butane to maleic anhydride.
2. Description of the Prior Art
Maleic anhydride is a well known and versatile intermediate for the manufacture of unsaturated polyester resins, chemical intermediates such as butanediol and tetrahydrofuran, pharmaceuticals and agrochemicals. It is produced by partial oxidation of aromatic (e.g., benzene) or non-aromatic (e.g., n-butane) hydrocarbons. The oxidation is performed in the gas phase, in the presence of a heterogeneous catalyst. The oxidation reaction may be carried out in a fixed, fluidized, or riser bed reactor.
Catalysts containing vanadium and phosphorus oxides have been used in the oxidation of 4-carbon atom hydrocarbons, such as n-butane, n-butenes, 1,3-butadiene or mixtures thereof with molecular oxygen or oxygen-containing gas to produce maleic anhydride. Conventional methods of preparing these catalysts involve reducing a pentavalent vanadium compound, and combining the same with a phosphorus compound, and if desired, promoter element compounds under conditions which will provide vanadium in a valence state below +5 to form catalyst precursors capable of being converted to vanadium phosphorus oxide. The catalyst oxide precursor is then recovered and converted to active catalytic material before or after the suitable catalyst particles for either fixed bed or fluid bed are formed.
The prior art describes many different procedures for this preparation, which in general involve the use of vanadium pentoxide (V2O5) as a source of vanadium (see e.g. U.S. Pat. No. 5,137,860 and EP 0 804 963 A1). Hydrogen chloride in aqueous solution is one of the reducing agents mentioned for the reduction of V+5 to V+4. Also used are organic reducing media like primary or secondary aliphatic alcohols or aromatic alcohols such as isobutyl alcohol and benzyl alcohol. The most used organic reducing agent is isobutyl alcohol since it combines optimal solvent and redox characteristics, thus favouring a complete redox reaction with formation of tetravalent vanadium, which is reacted with phosphoric acid to form vanadyl acid orthophosphate hemihydrate, (VO)HPO4.0.5H2O, which is then subject to further heat treatment to yield a finished catalyst.
U.S. Pat. Nos. 3,888,886; 3,905,914; 3,931,046; 3,932,305 and 3,975,300 disclose the testing of promoted vanadium phosphorus oxide catalysts for maleic anhydride production from butane in one inch diameter fluid bed reactors. In most instances, the catalysts were prepared by forming the vanadyl acid orthophosphate hemihydrate catalyst precursor in aqueous media (in U.S. Pat. No. 3,975,300 the precursor was formed in a paste of a vanadium compound, a phosphorus compound and an organic reducing agent), drying and thereafter grinding and sieving the precursor to a powder of about 74 to 250 microns size.
U.S. Pat. No. 4,647,673 discloses a process for the preparation of attrition resistant, microspheroidal fluid bed catalysts comprising the mixed oxides of vanadium and phosphorus in which a vanadium phosphorus mixed oxide catalyst precursor is densified, comminuted, formed into fluidizable particles and calcined under fluidization-type conditions.
The performance of catalyst comprising the mixed oxides of vanadium and phosphorus may be modified and can be substantially improved by the addition of a promoter element selected from the groups IA, IB, IIA, IIB, IIIA, IIIB, IVA, IVB, VA, VB, VIA, VIB and VIIIA of the periodic table of elements, or of mixtures of such elements. Patent literature claims that the catalytic performance of such catalyst can be substantially improved by addition of these elements. A review of the promoters reported in the literature and of their role has been reported by G. J. Hutchings in Appl. Catal., 1991, 72, 1-32, and in Stud. Surf. Sci. Catal. “Preparation of Catalysts VI”, (G. Poncelet et al., Eds.), Vol. 91, Elsevier Science, Amsterdam, 1995, p. 1. Other mention of promoters to improve the performance of catalyst comprising the mixed oxides of vanadium and phosphorus include V. Guliants et al. (Catalyst Letters 62 (1999), 87-9) wherein such catalysts were promoted with Nb, Si, Ti, and Zr; I. Mastuura, et al. (Catal. Today, 1996, 28, 133-138) wherein co-precipitate V and Nb are co-precipitated in an aqueous solution and then the precipitate is treated with benzyl alcohol at reflux; P. G. Pries de Oliveira, et al. (Catal. Today, 2000, 57, 177-186) wherein the VPO precursor is prepared in isobutyl alcohol and NbPO4 is introduced just before the nucleation of vanadyl acid orthophosphate hemihydrate; A. M. Duarte de Farias et al. (J. Catal. 2002, 208, 238-246) solubilize Nb ethoxide into isobutyl alcohol and use it as a reducing agent to prepare the Nb modified catalyst precursor and then the precursor is activated under reaction conditions; U.S. Pat. No. 4,147,661 to Higgins et al. wherein the Nb promoted catalyst is prepared in isobutyl alcohol using hydrogen chloride gas as reducing agent; U.S. Pat. No. 7,638,457 to Ghelfi et. al, wherein small amounts of Nb compounds or salts thereof are added in the preparation of a catalyst precursor mixture, which mixture includes a vanadium source, a phosphorus source, an organic medium capable of acting as a solvent and a reducing agent, and an additive selected from the group consisting of benzyl alcohol and polyols, followed by a thermal treatment of the precursor carried out in the presence of steam.